Stars are all big, hot, and heavy. But it takes a lot of work to determine just how big, how hot, and how heavy. We can't send out a probe to wrap a tape measure around a star's waist or put it on a bathroom scale. So astronomers must make all of these measurements remotely.

Consider the star Aldebaran, the bright orange "eye" of Taurus, the bull. It's well to the right of the Moon as they rise late this evening.

The star is more than 40 times wider than the Sun, thousands of degrees cooler, and about 70 percent more massive.

The mass is less well known than many other measurements for the star because astronomers have to calculate it using models. They plug in the star's spectral type, its size, and other factors, and the models give them a mass.

The most accurate measurements of mass come not from single stars like Aldebaran, but from binary stars -- two stars that are bound to each other by their mutual gravitational pull. Astronomers precisely plot the orbit of the two stars around their common center of gravity. Then they apply the laws of orbital mechanics to compute the stars' masses.

The masses of these and other binaries show that there's a direct relationship between a star's mass, size, and temperature That's what makes it possible to determine the masses of stars like Aldebaran -- stars that travel through the galaxy alone.

We'll talk about how astronomers measure a star's age tomorrow.

Script by Damond Benningfield, Copyright 2010

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